[daip] combining OTF maps in AIPS

[Eric Greisen]

Grace Wolf-Chase writes:

 > My colleague, JoAnn O'Linger, and I have several OTF maps of a very large
 > portion of a molecular cloud, and several smaller maps that were centered
 > on a subregion of the cloud. We also have a few incomplete maps of both
 > regions. So I have two questions:
 > 
 > (1) Is it possible (and advisable) to combine incomplete and complete OTF maps
 > of the same region in order to attain better S/N over at least part of the map?
 > If so, is it best to do this by reading the multiple maps into one OTFUV
 > file, or separately, into a number of OTFUV files, and then combine them at
 > a later stage of data reduction?

      It is best to read each in to separate OTFUV "uv" files to start
with.  Then correct each with SDVEL so that they are all on a
consistent LSR velocity system.  The 12m observes with various schemes
for updating frequency for position and time - I do not know what was
and is used now.  But the frequency does stay fixed for some time and
over a range of positions and so the spectra need to be shifted so
that, e.g. channel 43 is always 50 km/s LSR no matter what the time or
position.

      Then you may either image each separately and combine them
later with WTSUM or combine the "uv" data with DBCON and image them
together to begin with.  The two should be equivalent and the former
then lets you check each data set for possible bad data without
confusion about which data set contains the bad data.

 > 
 > (2) Is it possible to combine a larger OTF map with an OTF map
 > of a subregion that was centered at a different position? If so, how?

WTSUM was intended to do this.

Please read chapter 10 of the CookBook which covers much of this.
Get it from the web via
      http://www.aoc.nrao.edu/aips/cook.html
The chapter is also on your aips installation at
$AIPS_ROOT/TEXT/PUBL/COOK10.PS


Eric Greisen